Advances in nanotechnology make it possible to produce components with increasingly smaller structure elements. Photolithographic masks are often used for producing microstructured components or circuits. On account of the ever decreasing dimensions of the structure elements, said masks cannot always be produced without defects that are visible or printable on a wafer. Owing to the costly production of photolithography masks, photomasks or simply masks, defective photomasks are repaired, whenever possible. Charged particle beams such as, for instance, ion beams or electron beams in combination with a suitable gas or a gas mixture are typically used for this purpose. By way of example, this is explained in the article “Increasing mask yield through repair enhancement utilizing the MeRit®” by A. Garetto, J. Oster, M. Waiblinger and K. Edinger, 26th European Mask and Lithography Conference, Proc. SPIE, Vol. 7545, 75450H1-754450H9 and in WO 2010/072 279 A1. Said charged particles may lead to an electrical charging of a mask surface.
Besides charged particles, high-energy photon beams, for example extreme ultraviolet (EUV) radiation, that are used for the exposure of reflective EUV masks may also have an ionizing effect and lead to an electrical charging of a mask surface as a result. Furthermore, process steps when producing photomasks, such as etching processes, for instance, may lead to a charging of the mask surface. Moreover, the handling of a sample, for example of a wafer or of a photomask, may already lead to an electrostatic charging of the surface thereof. If the specimen is a wafer to be processed, coating processes and/or etching processes may also result in an electrical charging of the sample.
In the case of samples that have an electrically conductive surface, electrical charging can be avoided by earthing the sample. In the case of electrically insulating or semiconducting samples, surface charges can be prevented by vapor-depositing a thin conductive layer onto the surface of a sample to be examined. This last step may not be possible, however, for many applications, particularly if the samples to be analyzed are used in the production of, for example, microstructured semiconductor components or the production and/or repair of photomasks.
An electrostatic charging of a photomask or generally of a sample surface may obstruct subsequent examinations. By way of example, it may be necessary to analyze the surface of a photomask or of a sample with a scanning probe microscope such as, for instance, an atomic force microscope (AFM) or scanning force microscope (SFM). However, this may encounter difficulties since an electrostatic charging of the sample surface may corrupt a direct examination with a scanning probe microscope on account of the electrostatic interaction between the sample surface and a conductive measuring tip of the scanning probe microscope. This means that an electrical charging of the sample may make it impossible to analyze the topography thereof. Worse still, an electrical charging of the sample surface, in the course of the approach of a conductive measuring tip to the sample surface, may discharge like a flash (tip discharge), wherein the electrical charge of the sample surface may flow away via the measuring tip of the scanning probe microscope. The sudden discharge of the sample surface may lead to damage to a conductive measuring tip and/or a sample surface.
FIG. 1 shows the image of an excerpt from a photomask having damage that arose in the course of the approach of a conductive measuring tip of a scanning probe microscope to the electrostatically charged surface of the photomask. The image in FIG. 1 was recorded by a scanning electron microscope. The damage brought about by the electrostatic charging of the photomask is referred to as ESD damage (for electro-static damage). The damage shown in FIG. 1 can't be repaired and may thus lead to a loss of the illustrated photomask, which has to be produced anew in a time-consuming and costly process.
Besides the damage to sample surfaces as illustrated in FIG. 1, normally the measuring tips are altered by the high current density of the discharge current and have to be exchanged after an uncontrollable discharge of an electrostatically charged sample surface. (This is not shown in FIG. 1). To summarize, FIG. 1 illustrates that a discharge of a sample surface such as a flash via a conductive measuring tip of a scanning probe microscope must be prevented.
By use of a targeted discharge of an electrostatically charged surface of a sample, for example of a photomask, it is possible at least partly to prevent ESD damage during the use of a scanning probe microscope. One possibility for discharging a sample surface is to fit electrical contacts to the sample. However, this is of no help in the case of a local electrostatic charging, such as occurs on photomasks, for example, which preferably have a multiplicity of insulated electrically conductive structure elements fitted on an electrically insulating quartz substrate. Another possibility for discharging a sample surface with the aid of a corona discharge is disclosed in DE 10 2013 212 957 A1.
The discharge of a sample surface by the method mentioned requires, on the one hand, the additional incorporation of a corona discharge unit in the vicinity of that region of a sample which is intended to be examined by a measuring tip of a scanning probe microscope, or on the other hand—if the discharge is effected with the aid of radioactive substances—handling with these substances. Furthermore, it is difficult to control the discharge of a sample surface, and so local chargings may still be present on a sample surface even after a discharge process.
The examination of electrically charged surfaces with a scanning probe microscope has already been intensively investigated. The authors M. Nonnenmacher, M. P. O'Boyle and H. K. Wickramasinghe, Appl. Phys. Lett. 58, 2921 (1991) and also J. M. R. Weaver and D. W. Abraham, J. Vac. Sci. Techn., B9, 1159 (1991) describe a Kelvin force microscope that can be used to minimize the contribution of the electrostatic force to the oscillation frequency of the measuring tip. U.S. Pat. No. 5,308,974 discloses a method in which a sample to be examined is scanned once in contact with the sample surface in order to record topographical information and once using the topographical information recorded in the first scan, in order to separate the different force contributions acting on the measuring tip of a cantilever. In the article “High-resolution capacitance measurement and potentiometer by force microscopy”, the authors Y. Martin, D. A. Abraham and H. K. Wickramasinghe, Appl. Phys. Lett. 52, 1103, 1988, theoretically and experimentally investigate the behavior of the electrostatic interaction between an earthed sample and an electrostatically charged measuring tip of a force microscope in a one- and two-digit distance range between sample and measuring tip.
However, the cited documents do not appear to address the subject of an uncontrolled discharge of a charged sample surface via an electrically conductive measuring tip and the attendant ESD damage of the sample surface and/or of the measuring tip of the scanning probe microscope.
Therefore, the present invention addresses the problem of specifying a method and an apparatus which avoid damage when analyzing a charged sample surface with a scanning probe microscope.